Lithium-Manganese-Type Solid Solution Positive Electrode Material

ABSTRACT

Provided is a lithium-manganese-type solid solution positive electrode material capable of effectively suppressing gas generation in an initial cycle. Proposed is a lithium-manganese-type solid solution positive electrode material that includes a monoclinic structure of C2/m in a hexagonal structure of a space group R-3m. The lithium-manganese-type solid solution positive electrode material contains a solid solution expressed by a composition formula: xLi 4/3 Mn 2/3 O 2 +(1−x)LiMn α Co β Ni γ O 2  (in the formula, 0.2≦α≦0.6, 0≦β≦0.4, and 0.2≦γ≦0.6). In the composition formula, x is equal to or more than 0.36 and less than 0.50.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2012/053275, filed Feb. 13, 2012, and claims priority to JapanesePatent Application Nos. 2011-033071 filed Feb. 18, 2011, and 2011-143917filed Jun. 29, 2011, the disclosures of which are hereby incorporated intheir entirety by reference.

FIELD OF THE INVENTION

The present invention relates to a solid solution positive electrodematerial that may be used as a positive electrode active material of alithium battery such as a lithium primary battery, a lithium secondarybattery, a lithium ion secondary battery, and a lithium polymer battery,and particularly, to a lithium-manganese-type solid solution positiveelectrode material containing lithium and manganese.

DESCRIPTION OF RELATED ART

In a lithium battery, particularly, a lithium secondary battery, a massper unit quantity of electricity is small, and thus the energy densitythereof is high. Accordingly, the lithium secondary battery has beenspreading quickly as a driving power supply that is mounted on portableelectronic apparatuses such as a video camera, a notebook computer, anda cellular phone, or on electric vehicles.

A high energy density of the lithium secondary battery is mainlydependent of a potential of a positive electrode material, and as apositive electrode active material, in addition to lithium manganeseoxide (LiMn₂O₄) having a spinel structure, lithium composite oxides(LiMxOy) such as LiCoO₂, LiNiO₂, and LiMnO₂ which have a layer structurehave been known.

The positive electrode active material used in the majority of lithiumsecondary batteries that currently come into the markets is LiCoO₂having a high voltage of 4 V. Since Co is significantly expensive, as asubstitution material of LiCoO₂, for example, LiFePO₄,LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂, and the like may be included, and researchon the substitution materials is in progress.

In recent years, it has been reported that a solid solution of LiMO₂ andLi₂MnO₃ used as the positive electrode active material exhibits a highcapacity nearly double the capacity of LiCoO₂ in a case of being chargedup to 4.5 V or more (Non-Patent Document 1), and that so-called solidsolution positive electrode material has attracted attention.

With regard to the solid solution positive electrode material, forexample, Patent Document 1 discloses a positive electrode material for alithium ion battery which is expressed by a general formula:xLiMO₂·(1−x)Li₂NO₃ (here, x represents a number satisfying 0<x<1; Mrepresents one or more transition metals of which an average oxidationstate is 3⁺; and N represents one or more transition metals of which anaverage oxidation state is 4⁺), and is subjected to an oxidationtreatment.

In addition, Patent Document 2 discloses a positive electrode for alithium ion battery, wherein a main active material of the positiveelectrode is expressed by a general formula:xLi₂MO₃·(1−x)Li[Ni_(1−y−z)Co_(y)A_(z)]O₂ (here, x represents a numbersatisfying 0.4<x<1.0; M represents one or more elements selected from agroup consisting of Mn, Ti, and Zr; A represents one or more elementsselected from a group consisting of B, Al, Ga, and In; 0<y≦0.3; and0<z≦0.1).

CITATION LIST Non-Patent Document

Non-Patent Document 1: A. Ito, D. Li, Y. Ohsawa, Y. Sato, J PowerSources, 183, 344 (2008)

Patent Document

Patent Document 1: JP 2008-270201 A

Patent Document 2: JP 2010-108873 A

Among the above-described solid solution positive electrode materials,the lithium-manganese-type solid solution positive electrode materialhas been regarded as a promising material from the viewpoints that adischarge potential is high, a high energy density may be expected, andthe raw material cost may be reduced because Mn is used as a base.However, this kind of lithium-manganese-type solid solution positiveelectrode material has a serious problem in that a gas is generated whenbeing charged and discharged, particularly, in an initial cycle.

Therefore, the invention provides a new lithium-manganese-type solidsolution positive electrode material capable of effectively suppressinggas generation in an initial cycle.

SUMMARY OF THE INVENTION

The present inventors suggest a lithium-manganese-type solid solutionpositive electrode material that includes a monoclinic structure of C2/min a hexagonal structure of a space group R-3m. Thelithium-manganese-type solid solution positive electrode materialcontains a solid solution expressed by a composition formula:xLi_(4/3)Mn_(2/3)O₂+(1−x)LiMn_(α)Co_(β)Ni_(γ), O₂ (in the formula,0.2≦α≦0.6, 0≦β≦0.4, and 0.2≦γ≦0.6). In the composition formula, x isequal to or more than 0.36 and less than 0.50.

In the solid solution of Li_(4/3)Mn_(2/3)O₂ and LiMn_(α)Co_(β)Ni_(γ)O₂(in the formula, 0.2≦α≦0.6, 0≦β≦0.4, and 0.2≦γ≦0.6), when the solidsolution is produced by significantly narrowing the x, representing amolar ratio between Li_(4/3)Mn_(2/3)O₂ and LiMn_(α)Co_(β)Ni_(γ)O₂, to beequal to or more than 0.36 and less than 0.50, it is proved that gasgeneration in an initial cycle is significantly suppressed. Accordingly,the solid solution positive electrode material may be used for alarge-sized battery that was not realized when use common type of solidsolution positive electrode material.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an XRD pattern of a solid solution powder (sample) which isobtained in Example 2;

FIG. 2 is a graph illustrating composition dependency of an amount ofgas generation based on measured values of Examples 1 to 5, andComparative Examples 1 and 2; and

FIG. 3 is a graph illustrating the composition dependency of the amountof gas generation based on measured values of solid solution powders(samples) which are obtained in Examples 2, 6, and 7.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Next, the invention will be described based on embodiments, but theinvention is not limited to the embodiments to be described below.

<Composition of Present Solid Solution Positive Electrode Material>

A solid solution positive electrode material according to the embodiment(hereinafter, referred to as “present solid solution positive electrodematerial”) is a positive electrode material which includes a monoclinicstructure of C2/m in a hexagonal structure of a space group R-3m, andwhich contains a solid solution expressed by a composition formula:xLi_(4/3)Mn_(2/3)O₂+(1−x)LiMn_(α)Co_(β)Ni_(γ)O₂ (in the formula,0.2≦α≦0.6, 0≦β≦0.4, and 0.2≦γ≦0.6).

As can be seen also from an XRD pattern illustrated in FIG. 1, thepresent solid solution positive electrode material has a peak which doesnot belong to the hexagonal structure of the space group R-3m, butbelongs to the monoclinic structure of C2/m at 2θ=19.5 to 23.0° inaddition to a peak that belongs to the hexagonal structure of the spacegroup R-3m. This represents that the present solid solution positiveelectrode material has the monoclinic structure of C2/m in the hexagonalstructure of the space group R-3m.

In the present solid solution positive electrode material, it isimportant that x, representing a molar ratio between Li_(4/3)Mn_(2/3)O₂and LiMn_(α)Co_(β)Ni_(γ)O₂ (in the formula, 0.2≦α≦0.6, 0≦β≦0.4, and0.2≦γ≦0.6), is equal to or more than 0.36 and less than 0.50. When x isequal to or more than 0.36 and less than 0.50, gas generation in aninitial cycle may be significantly suppressed. From the viewpoint ofcapable of suppressing the gas generation, x is more preferably 0.38 to0.48, and still more preferably 0.40 to 0.47.

In addition, as the above-described LiMn_(α)Co_(β)Ni_(γ)O₂ (in theformula, 0.2≦α≦0.6, 0≦β≦0.4, and 0.2≦γ≦0.6), a composition formulaexpressed by LiMn_((1−β)/2)Co_(β)Ni_((1−β)/2)O₂ is more preferable. Atthis time, when β that specifies a molar ratio of Mn, Co, and Ni is 0.2or less, gas generation may be effectively suppressed. In addition, βmay be 0.0, and β may be within a range of 0.0 to 0.2.

In addition, in the composition formula, as a substitution element ofthe metal elements other than Li, that is, Mn, Co, and Ni, one kind ortwo kinds selected from a group consisting of Nb, V, Mg, Al, and Ti maybe contained. However, the total content of the substitution element ispreferably 10% by mole or less of the number of moles of the metalelements other than Li. When the total substitution elements content is10% by mole or less, it is considered that the same effect as a case inwhich the substitution elements are not contained may be obtained.

Furthermore, the present solid solution positive electrode material mayfurther contain B (boron). At this time, B (boron) may be present in thesolid solution, or may be present outside the solid solution.

From the viewpoint of effectively suppressing the gas generation, in thepresent solid solution positive electrode material, a c-axis length of acrystal lattice in the hexagonal structure of the space group R-3m ispreferably 14.255 Å to 14.275 Å, more preferably 14.257 A to 14.274 Å,and still more preferably 14.259 Å to 14.271 Å.

In addition, similarly, from the viewpoint of effectively suppressingthe gas generation, in the present solid solution positive electrodematerial, it is preferable that in an XRD pattern that is obtained byXRD measurement, a ratio of the sum area of peaks located within a rangeof θ=19.5° to 23.5° to the total area obtained by adding the sum area ofpeaks located within the range of θ=18.0° to 19.5° and the sum area ofpeaks located within a range of θ=43.0° to 46.0° is 0.145 to 0.185, morepreferably 0.148 to 0.182, and still more preferably 0.151 to 0.179.

The peaks located within the range of θ=18.0° to 19.5° correspond todiffraction peaks derived from a (003) plane of a space group R-3m andpeaks derived from a (001) plane of a space group C2/m. The peakslocated within the range of θ=43.0° to 46.0° correspond to diffractionpeaks derived from a (104) plane of the space group R-3m, and peaksderived from a (022) plane, a (220) plane, a (−202) plane, and a (131)plane of the space group C2/m. The peaks located within the range ofθ=19.5° to 23.5° correspond to diffraction peaks derived from a (020)plane and a (110) plane of the space group C2/m. Accordingly, the peakarea ratio correlates with the amount of the crystal structure of thespace group C2/m with respect to the entirety of crystal structuresincluding the space groups R-3m and C2/m, and thus the content of thecrystal structure of the space group C2/m with respect to the entiretyof the crystal structures may be examined by examining the peak arearatio.

<Method of Producing Present Solid Solution Positive Electrode Material>

First, for example, raw materials such as a lithium salt compound, amanganese salt compound, a nickel salt compound, and a cobalt saltcompound are mixed with each other, and the resultant mixture ispulverized by a wet-type pulverizer or the like. Then, the resultantpulverized mixture is granulated and dried by a spray drier or the like,and is burned. The resultant burned material is classified, or milledusing a classifier-attached collision-type mill if necessary. Inaddition, a heat treatment and a subsequent classification are performeddepending on the circumstance, whereby the present solid solutionpositive electrode material may be obtained.

However, the method of producing the present solid solution positiveelectrode material is not limited to the above-described productionmethod. For example, a granulated powder, which is subjected to theburning, may be prepared by using the generally called coprecipitationmethod.

Examples of the lithium salt compound include lithium hydroxide (LiOH),lithium carbonate (Li₂CO₃), lithium nitrate (LiNO₃), lithium hydroxidehydrate (LiOH·H₂O), lithium oxide (Li₂O), fatty acid lithium, lithiumhalide, and the like. Among these, the lithium hydroxide, the lithiumcarbonate, and the lithium nitrate are preferable.

The manganese salt compound is not particularly limited. For example,manganese carbonate, manganese nitrate, manganese chloride, manganesedioxide, and the like may be used. Among these, manganese carbonate andmanganese dioxide are preferable. In the manganese salt compounds,electrolytic manganese dioxide that may be obtained according to anelectrolytic method is particularly preferable.

The kind of the nickel salt compound is not particularly limited. Forexample, nickel carbonate, nickel nitrate, nickel chloride, nickeloxyhydroxyde, nickel hydroxide, nickel oxide, and the like may be used.Among these, nickel carbonate, nickel hydroxide, and nickel oxide arepreferable.

The kind of the cobalt salt compound is not particularly limited. Forexample, basic cobalt carbonate, cobalt nitrate, cobalt chloride, cobaltoxyhydroxide, cobalt hydroxide, cobalt oxide, and the like may be used.Among these, the basic cobalt carbonate, the cobalt hydroxide, thecobalt oxide, and the cobalt oxyhydroxide are preferable.

When mixing the raw materials, it is preferable to add a liquid mediumsuch as water and a dispersant to the raw materials to perform wet-typemixing for slurrying of the raw materials. In addition, the obtainedslurry is preferably to be milled using a wet-type mill. However, a drymilling can be also used.

In addition, it is preferable that average particle size (D50) becomesfrom 0.2 μm to 1.0 μm after milling.

A granulation method may be a wet-type method or a dry-type method aslong as various raw materials that are milled in the precedent processare not separated and are dispersed in a granulated particle. Examplesof the granulation method include an extruding granulation method, arolling granulation method, a fluidized bed granulation method, a mixinggranulation method, a spray drying granulation method, a pressuremolding granulation method, and a flake granulation method using a rollor the like. However, in the case of the wet-type granulation, it isnecessary to sufficiently dry the raw materials before burning. Examplesof a dry method include dry methods in the related art such as a spraythermal dry method, a hot-air dry method, a vacuum dry method, and afreeze-dry method. Among these, the spray thermal dry method ispreferable. The spray thermal dry method is preferably performed using aspray drier.

The burning is preferably performed in a burning furnace under an airatmosphere, an atmosphere in which an oxygen partial pressure isadjusted, a carbon dioxide gas atmosphere, or other atmospheres in sucha manner that retention is performed for 0.5 hours to 30 hours at atemperature of 850° C. to 1,100° C. (representing a temperature in acase of bringing a thermocouple into contact with a burned product in aburning furnace). At this time, it is preferable to select burningconditions in which a transition metal is solid-soluted in an atom leveland shows a single phase.

The kind of the burning furnace is not particularly limited. Forexample, the burning may be performed using a rotary kiln, astill-standing furnace, or other burning furnaces.

Classification after the burning has a technical meaning of adjusting aparticle size distribution of an agglomerated powder and of removingforeign matters. In addition, it is preferable to perform theclassification to obtain an average particle size (D50) from 2 μm to 50μm.

It is preferable that the milling after the classification is performedto realize fine milling using a classifier-attached collision-type mill,for example, a classification-rotor-attached counter jet mill in such amanner that a ratio between the average particle size (D50) and acrystallite diameter is within a predetermined range.

Powder particles, obtained by milling using the classifier-attachedcollision-type mill commonly have a not-spherical shape.

In the heat treatment, it is preferable that retention is performedunder an air atmosphere for 0.5 hours to 20 hours under an environmentof 300° C. to 700°, and more preferably 600° C. to 700° C. At this time,at a temperature lower than 300° C., there is a concern that it isdifficult to obtain the heat treatment effect, and thus the fine powderremains not sintered. On the other hand, when the heat treatment isperformed at a temperature higher than 700° C., sintering starts, andthus powder characteristics of the invention may not be obtained.

The classification after the heat treatment has a technical meaning ofadjusting a particle size distribution of an agglomerated powder and ofremoving foreign matters. In addition, it is preferable to perform theclassification in an average particle size (D50) of 2 μm to 50 μm.

<Usage of Present Solid Solution Positive Electrode Material>

The present solid solution positive electrode material may beeffectively used as a positive electrode active material of lithiumbatteries after being crushed and classified as necessary.

For example, the present solid solution positive electrode material, aconducting material comprised of carbon black and the like, and a bindercomprised of Teflon (registered trademark) binder and the like are mixedwith each other to prepare a positive electrode mixture. In addition, alithium battery may be constructed by using the positive electrodemixture that is obtained as described above, lithium or a material suchas carbon capable of intercalating and deintercalating lithium as anegative electrode, and a material, which is obtained by dissolving alithium salt such as lithium hexafluorophosphate (LiPF6) in a mixedsolvent such as ethylene carbonate and dimethyl carbonate, as anon-aqueous electrolyte.

The lithium battery that is constituted as described may be used for adriving power supply that is mounted on electronic apparatuses such as anotebook computer, a cellular phone, a cordless phone extension, a videomovie, a liquid crystal television, an electric shaver, a portableradio, a headphone stereo, a backup power supply, and a memory card,medical equipment such as a pacemaker and a hearing aid, and electricvehicles. Among these, the lithium battery is particularly useful as adriving power supply of various kinds of portable computers such as acellular phone, a PDA (Personal Digital Assistant), and a notebookcomputer, an electric vehicle (including a hybrid vehicle), a powerstorage power supply, and the like in which excellent cyclecharacteristics are demanded.

<Explanation of Phrase>

In the present specification, description of “X to Y” (X and Y arearbitrary numbers) includes a meaning of “equal to or more than X andequal to or less than Y” and meaning of “preferably larger than X” or“preferably smaller than Y” unless otherwise stated.

In addition, description of “X or more” (X is an arbitrary number) or “Yor less” (Y is an arbitrary number” also includes intension of“preferably larger than X” or “preferably smaller than Y”.

EXAMPLES

Next, the invention will be further described with reference to examplesand comparative examples that are practically produced, but theinvention is not limited to the following examples.

<Battery Evaluation>

A laminate-type battery was prepared using a solid solution powder(sample) produced in examples and comparative examples as a positiveelectrode active material. The following gas generation evaluation testand battery performance evaluation test were performed using thebattery.

(Preparation of Laminate-Type Battery)

90% by weight of the solid solution powder (sample) produced in examplesand comparative examples as a positive electrode active material, 5% byweight of acetylene black as a conducting agent, 5% by weight of PVDF(polyvinylidene fluoride) as a binding agent were mixed with each other,and NMP (N-methyl pyrrolidone) was added to the resultant mixture toprepare a paste. The paste was applied on an aluminum foil currentcollector having a thickness of 15 μm, and was dried at 100° C. Then,the resultant positive electrode current collector on which the pastewas applied was compressed to have a thickness of 80 μm to prepare apositive electrode sheet.

Copper foil having a thickness of 18 μm was used as a negative electrodecurrent collector. 92% by weight of graphite as an active material and8% by weight of PVDF as a binding agent were mixed with each other, andNMP was added to the resultant mixture to prepare paste. The paste wasuniformly applied on the negative electrode current collector, and wasdried at 100° C. Then, the resultant negative electrode currentcollector on which the paste was applied was compressed to have athickness of 80 μm to prepare a negative electrode sheet.

The positive electrode sheet that was obtained as described above wascut to a size of 2.9 cm×4.0 cm, and was used as a positive electrode. Inaddition, the negative electrode sheet that was obtained as describedabove was cut to a size of 3.1 cm×4.2 cm, and was used as a negativeelectrode. A separator (formed from porous polyethylene film) wasinterposed between the positive electrode and the negative electrode toprepare a laminate-type battery. A separator to which an electrolyte wasimpregnated was used as the separator. The electrolyte was obtained bydissolving LiPF6 in a mixed solvent of ethylene carbonate, ethyl methylcarbonate, and dimethyl carbonate (volume ratio=20:20:60) to have aconcentration of 1 mol/liter, and by further adding 2% by volume ofvinylene carbonate as an additive to the resultant solution.

(Gas Generation Evaluation Test)

The laminate-type battery that was prepared by the above-describedmethod was left as is for 12 hours, and the laminate-type battery wascharged with a current density of 0.2 CmA/cm² until a potentialdifference between both electrodes became 4.8 V. Then, thelaminated-type battery was discharged with 0.2 CmA/cm² until thepotential difference became 1.9 V. Next, with the same current value, acycle of charging until the potential difference between both electrodesbecame 4.5 V and discharging until the potential difference became 1.9 Vwas performed 99 times.

An amount (mL) of swelling generated up to this time was measured by animmersion volumetric method (solvent substitution method based onArchimedes' principle).

Each results of Table 1 represent the total amount obtained byperforming measurement to two laminate-type batteries.

(Battery Performance Evaluation Test)

The following charging and discharging was performed using thelaminate-type battery prepared as described above to obtain a capacityretention ratio of each cycle compared to initial capacity.

A charging and discharging voltage range was set to 1.9 V to 4.8 V inthe first cycle, and 1.9 to 4.5 V from the second cycle. Inconsideration of the content of the positive electrode active materialin the positive electrode, a current value was obtained and was set inorder for a charging and discharging rate to be 0.2 C. The charging anddischarging was repeated under these conditions to calculate thecapacity retention rate in each cycle with respect to an initialdischarge capacity. A measurement temperature was set to 25° C.

<XRD Measurement>

The XRD measurement was performed under the following conditions usingan XRD measurement device (device name: RINT-TTR III, manufactured byRigaku Corporation) to obtain an XRD pattern, and a peak area wasobtained based on the XRD pattern.

A “peak area ratio” in table was calculated as a ratio of the sum area(the former/the latter) of peaks located within a range of θ=19.5° to23.5° to the total area obtained by adding the sum area of peaks locatedwithin the range of θ=18.0° to 19.5° and the sum area of peaks locatedwithin a range of θ=43.0° to 46.0° by analyzing the obtained XRDpattern. In addition, background removal was not performed during thecalculation.

=XRD Measurement Condition=

X-ray source: CuKα, operation axis: 2θ/θ, measurement method:continuous, and counting unit: cps

Initiation angle: 5°, and termination angle: 80°

Sampling width: 0.02°, and scanning speed: 4°/min

Voltage: 50 kV, and current: 300 mA

Divergence slit; ⅔°, and divergence vertical length: 10 mm

Scattering slit: ⅔°, and light-receiving slit: 0.15 mm

With regard to measurement of the c-axis length, refinement of a latticeconstant was performed using analysis software “JADE version 7.5.22(Japanese version)” to obtain the c-axis length.

Example 1

Lithium carbonate having an average particle size (D50) of 8 μm,electrolytic manganese dioxide having an average particle size (D50) of22 μm, cobalt oxyhydroxide having an average particle size (D50) of 14μm, and nickel hydroxide having an average particle size (D50) of 25 μmwere weighed in such a manner that a molar ratio of Li:Mn:Co:Ni becomes1.143:0.572:0:0.285, and were mixed and stirred with water addedthereto, whereby slurry having a solid content concentration of 50% byweight was prepared.

Ammonium polycarboxylate (SN dispersant 5468, manufactured by SAN NOPCOLIMITED) as a dispersant was added to the obtained slurry (raw materialpowder: 20 kg) in a content of 6% by weight based on the slurry solidcontent, and the resultant mixture was milled using a wet-type mill at1,300 rpm for 29 minutes to set the average particle size (D50) to 0.7μm.

The obtained milled slurry was granulated and dried using a spray drier(OC-16, manufactured by OHKAWARA KAKOHKI CO., LTD.). Granulation anddrying were performed using a rotary disk for spraying, the diskrotation was adjusted to 21,000 rpm, a slurry supply amount to 24 kg/hr,and an exit temperature of a dry column was adjusted to 100° C.

The obtained granulated powder was burned using a static type electricfurnace in the air at 950° C. for 20 hours. The burned powder that wasobtained after the burning was classified using a sieve having anopening of 75 μm to obtain a lithium-manganese-type solid solutionpowder (sample).

Examples 2 to 7, and Comparative Examples 1 and 2

A lithium-manganese-type solid solution powder (sample) was prepared inthe same manner as Example 1 except that the composition of the rawmaterial was changed.

<XRD Measurement and ICP Emission Spectrometry>

The lithium-manganese-type solid solution powders (samples) that wereobtained in Examples 1 to 7, and Comparative Example 1 and 2 wereanalyzed by XRD measurement. In the XRD measurement, for example, asillustrated in FIG. 1, peaks that were not derived from R-3m andbelonged to C2/m were recognized at 2θ=19.5° to 23.0°. Accordingly, itwas confirmed that the lithium-manganese-type solid solutions that wereobtained in Examples 1 to 7 and Comparative Examples 1 and 2 includeC2/m monoclinic structure in a hexagonal structure of a space groupR-3m, and the solid solution is a solid solution of Li_(4/3)Mn_(2/3)O₂and LiMn_(1/2)Ni_(1/2)O₂.

In addition, the composition analysis of the lithium-manganese-typesolid solution powders (samples) that were obtained in Examples 1 to 7and Comparative Examples 1 and 2 was performed by ICP emissionspectroscopy, and compositions in Table 1 were found.

TABLE 1 Capacity retention Metal molar rate of Lattice Amount of ratiocalculated laminate constant Peak gas generation from ICP analysisresults xLi_(4/3)Mn_(2/2)O₂ + cell after c- area of two laminate cells(Me is fixed to 2) (1 − x)LiMn_(α)Co_(β)Ni_(γ)O₂ 100 cycles axis ratioafter 100 cycles [cc] Li Mn Co Ni α β γ x [%] [Å] [—] Example 1 Lessthan measurement 1.130 0.588 0.000 0.278 0.5 0.0 0.5 0.43 77.9 14.2710.167 limit Example 2 Less than measurement 1.140 0.593 0.000 0.270 0.50.0 0.5 0.45 76.5 14.269 0.162 limit Example 3 Less than measurement1.140 0.591 0.000 0.261 0.5 0.0 0.5 0.47 77.1 14.266 0.170 limit Example4 Less than measurement 1.150 0.535 0.000 0.318 0.5 0.0 0.5 0.36 78.214.274 0.156 limit Example 5 1.0 1.190 0.561 0.000 0.247 0.5 0.0 0.50.49 85.8 14.256 0.185 Example 6 Less than measurement 1.190 0.536 0.0300.245 0.455 0.09 0.455 0.45 73.2 14.264 0.146 limit Example 7 1.0 1.1600.526 0.060 0.252 0.41 0.18 0.41 0.45 79.5 14.258 0.163 ComparativeExample 1 1.5 1.090 0.573 0.000 0.337 0.5 0.0 0.5 0.30 71.1 14.280 0.140Comparative Example 2 2.5 1.190 0.603 0.000 0.205 0.5 0.0 0.5 0.60 83.314.252 0.189

From the results of Table 1, FIG. 2, and FIG. 3, with regard to thelithium-manganese-type solid solution powders (samples) that wereobtained in Examples 1 to 7, gas generation was sufficiently suppressed.Among these, with regard to the lithium-manganese-type solid solutionpowders (samples) that were obtained in Examples 1 to 4, and Example 6,gas generation was not recognized (less than a measurement limit).

From these results, in a positive electrode material containing a solidsolution expressed by a composition formula:xLi_(4/3)Mn_(2/3)O₂+(1−x)LiMn_((1−β)/2)Co_(β)Ni_((1−β)/2)O₂ (β=0 to0.2), it could be seen that when x is equal to or more than 0.36 andless than 0.50, the gas generation may be significantly suppressed.

When putting together the results and testing experience until now, in apositive electrode material containing solid solution expressed by acomposition formula: xLi_(4/3)Mn_(2/3)O₂+(1−x)LiMn_(α)Co_(β)Ni_(γ)O₂ (inthe formula, 0.2≦α≦0.6, 0≦β≦0.4, and 0.2≦γ≦0.6), it may be consideredthat when x is equal to or more than 0.36 and less than 0.50, the gasgeneration is significantly suppressed similarly to the above-describedconditions.

In addition, with regard to the examples and comparative examples, inLiMn_((1−β)/2)Co_(β)Ni_((1−β)/2)O₂, β=0.0 to 0.18 was shown. In othertests not described in the present specification, it was confirmed thatwhen β is up to the extent of 0.4, the gas generation is within apermissible range, and when β is 0.2 or less, the gas generation may befurther suppressed. From these results, when suppressing of the gasgeneration may be confirmed in a case of β=0, it is considered that thegas generation may be suppressed to the extent of β=0.0 to 0.4.

1. A lithium-manganese-type solid solution positive electrode materialincluding a monoclinic structure of C2/m in a hexagonal structure of aspace group R-3m, wherein the lithium-manganese-type solid solutionpositive electrode material comprises a solid solution expressed by acomposition formula:xLi_(4/3)Mn_(2/3)O₂+(1−x)LiMn_(α)Co_(β)Ni_(γ)O₂ (in the formula,0.2≦α≦0.6, 0≦β≦0.4, and 0.2≦γ≦0.6); and in the composition formula, x isequal to or more than 0.36 and less than 0.50.
 2. Alithium-manganese-type solid solution positive electrode materialincluding a monoclinic structure of C2/m in a hexagonal structure of aspace group R-3m, wherein the lithium-manganese-type solid solutionpositive electrode material comprises a solid solution expressed by acomposition formula:xLi_(4/3)Mn_(2/3)O₂+(1−x)LiMn_((1−β)/2)Co_(β)Ni_((1−β)/2)O₂ (in theformula, β is 0 to 0.2); and in the composition formula, x is equal toor more than 0.36 and less than 0.50.
 3. The lithium-manganese-typesolid solution positive electrode material according to claim 2, whereinin the composition formula, β is
 0. 4. The lithium-manganese-type solidsolution positive electrode material according to claim 1, wherein in anXRD pattern, in addition to a peak that belongs to the hexagonalstructure of the space group R-3m, a peak which does not belong to thehexagonal structure of the space group R-3m but belongs to themonoclinic structure of C2/m is present at 2θ=19.5 to 23.0°.
 5. Thelithium-manganese-type solid solution positive electrode materialaccording to claim 1, wherein a c-axis length of a crystal lattice inthe hexagonal structure of the space group R-3m is 14.255 Å to 14.275 Å.6. The lithium-manganese-type solid solution positive electrode materialaccording to claim 1, wherein in an XRD pattern that is obtained by XRDmeasurement, a ratio of the sum area of peaks located within a range ofθ=19.5° to 23.5° to the total area obtained by adding the sum area ofpeaks located within a range of θ=18.0° to 19.5° and the sum area ofpeaks located within a range of θ=43.0° to 46.0° is 0.145 to 0.185. 7.The lithium-manganese-type solid solution positive electrode materialaccording to claim 1, wherein in the composition formula, as asubstitution element of the metal elements other than Li, one or twokinds selected from a group consisting of Nb, V, Mg, Al, and Ti arecontained, and the total content of the substitution element is 10% bymole or less of the number of moles of the metal elements other than Li.8. The lithium-manganese-type solid solution positive electrode materialaccording to claim 1, wherein the lithium-manganese-type solid solutionpositive electrode material further contains B (boron).
 9. Thelithium-manganese-type solid solution positive electrode materialaccording to claim 2, wherein in an XRD pattern, in addition to a peakthat belongs to the hexagonal structure of the space group R-3m, a peakwhich does not belong to the hexagonal structure of the space group R-3mbut belongs to the monoclinic structure of C2/m is present at 2θ=19.5 to23.0°.
 10. The lithium-manganese-type solid solution positive electrodematerial according to claim 2, wherein a c-axis length of a crystallattice in the hexagonal structure of the space group R-3m is 14.255 Åto 14.275 Å.
 11. The lithium-manganese-type solid solution positiveelectrode material according to claim 2, wherein in an XRD pattern thatis obtained by XRD measurement, a ratio of the sum area of peaks locatedwithin a range of θ=19.5° to 23.5° to the total area obtained by addingthe sum area of peaks located within a range of θ=18.0° to 19.5° and thesum area of peaks located within a range of θ=43.0° to 46.0° is 0.145 to0.185.
 12. The lithium-manganese-type solid solution positive electrodematerial according to claim 2, wherein in the composition formula, as asubstitution element of the metal elements other than Li, one or twokinds selected from a group consisting of Nb, V, Mg, Al, and Ti arecontained, and the total content of the substitution element is 10% bymole or less of the number of moles of the metal elements other than Li.13. The lithium-manganese-type solid solution positive electrodematerial according to claim 2, wherein the lithium-manganese-type solidsolution positive electrode material further contains B (boron).